1. Field of Invention
This invention relates to facet-based, compact, and high-efficiency optical concentrators, collimators, reflectors, and couplers that do not, in general, preserve the point-to-point ordering of the input light rays so that maximum power transfer from a first aperture to a second aperture is achieved.
2. Prior Art
The physical processes that are available to the designer of a light concentrator are limited. Following what has become convention in the field of optical concentration we shall give letter designations to different processes used for controlling light: surface Refraction (R), surface reflection (X), total Internal reflection (I), Diffraction (D), volume refraction due to Gradients in refractive index (G). A concentrator is then classified by the primary mechanisms that are used to implement the concentration. Secondary processes comprising less than 50% of the optical interactions are typically not listed.
For example, a simple concentrating lens having two refractive surfaces with a constant refractive index in between the surfaces is described as an RR (or R2) concentrator; while a cassegrainian-like two mirror concentrating system that is immersed in air is described as an XX (or X2) concentrator system. The ordering of the letters, as read from left to right, is important and generally describes the order of interaction of the processes involved in the concentration of light. Therefore, a concentrator described as RX is not the same as a concentrator described as XR because the RX concentrator involves a refraction followed by a reflection while the XR concentrator involves a reflection followed by a refraction for concentration. Even if there are only two physical surfaces it may also be possible to have three or more processes of light redirection involved. For example it is possible for an RXI concentrator to use refraction, followed by a mirror reflection, and finally total internal reflection while only using two physical surfaces. Note, that this nomenclature does not distinguish between different sources of radiant energy nor does it consider the end use of the light. Also note that due to time-reversal invariance of the equations of electromagnetics a concentrator may be run “backwards” as an expander or collimator of light (the opposite of a concentrator) such as might be useful for illumination applications from a small lamp, however, the left-to-right ordering of the categorizing letters is always meant to imply concentration in this document. With this systematic notation we can easily categorize the types of solar concentrators that exist in the prior art and then compare them to the current invention.
A canonical problem in the area of non-imaging optics is how to design ultra-efficient concentrators, collimators, and other power transfer devices, which are manufacturable, compact, robust, cost effective, and producible in large volumes. There are a number of concentration techniques that are available in the literature. However, most of these techniques use a two-parameter bundle of input rays for their design. In contradistinction, the present invention uses the full extended phase space for the design. In particular, the present invention deals predominantly with concentrators typically of type I2, I3, I4, and IN (where usually N≧2 and an integer).
The following are a list of the issued patents that use a variety of different techniques for concentration. It is observed that these patents, clearly do not anticipate, teach, or show in any way, the use of a large-area non-planar facet surface morphology for concentrating light by predominantly Total Internal Reflections.
U.S. Pat. No. 4,120,565 issued to Arnulf Rabl and Veronika Rabl on 1978 Oct. 17 deals with concentrators of type I2, which only use two total internal reflection redirections. However, this prior art addresses only a very specific from of optical facet having both a right-angle at each facet apex and linear flat surfaces on the facet sides. Although these concentrators are based on total internal reflections they are also substantially in error with respect to what physics is needed to actually implement a high concentration lens design. In particular, this prior art will always have a concentration that is strictly less than the theoretical limit imposed by physics because it has intrinsic astigmatism due to the strict use of right angles and flat surface facets in their patent. The resulting astigmatism is seen in FIG. 9. Finally, this prior art is also unable to provide any shaping of the resulting focused light spot.
U.S. Pat. Nos. 4,337,759; 5,404,869; 5,577,492; 5,577,493; 5,613,769; 5,676,453 and 5,655,832 issued in various combinations to John M. Popovich, William A. Parkyn Jr., and David G. Pelka deal with concentrators of type I, IR, and IR2 having large numbers of relatively small facets compared to the diameter of the lens. The non-imaging lenses they consider are also limited by the extreme sharp or acute angles of the facets making it very hard to manufacture. Additionally, the approaches presented in their patent are not capable of reaching the highest possible concentration because the focal region is typically placed in the air not the dielectric so that the advantages of a refractive index greater than unity are not exploited.
U.S. Pat. No. 6,252,155 issued to Ugar Ortabasi on Jun. 6, 2001 deals with concentrators of type RGXGX, which are based on the classical compound parabolic concentrator and are not capable of being made physically compact and also require multiple distinct parts for fabrication instead of just one compact transparent dielectric part.
U.S. Pat. No. 6,639,733 issued to Juan C. Minano, Pablo Benitez, Juan C. Gonzalez, Waqidi Falicoff, and H. J. Caulfield of Light Prescriptions Innovators LLC on Oct. 28, 2003 deals with concentrators of type RR, RX, RXI, X2, and XR. The concentrators of this prior art invention are limited by being restricted to: optical surfaces having continuous second derivatives; small or micro-structured facets; flat facets; facets that are based on complimentary pairs or sub-facets that must include both an active facet and an inactive facet; facets that are characterized by a deflection laws based on reflection from a mirrored facet surface, refraction from a refractive facet surface, and a combination of reflection and refraction from facets having mirrored and refractive surfaces; an extended phase-space design restricted to two spatial coordinates and one momentum coordinate for the input light bundle of rays as well as the design of the concentrator; and facets that are configured to exist only as concentric annuli around the optical axis. In all distinguishing cases just listed the present invention is different and teaches a different means to achieve concentration and collimation of light. In particular, the present invention deals predominantly with concentrators typically of type I2, I3, I4, and IN (where usually N≧2 and an integer). In general, the present invention uses at least one surface that has discontinuous second derivatives at periodic or non-periodic coordinate locations; large facets; curved facets; complimentary pairs of facets that only have active facet faces; facets that are characterized by a deflection law based on reflection from unmirrored facet surfaces using only total internal reflection; a design that demands working in a five dimensional extended phase space even if the input bundle of rays is restricted to two spatial coordinates and one momentum coordinate for the input bundle of rays; and each facets is configured to exist predominantly over a limited range of azimuthal angles Δφ instead of 360 degree for the prior art.
U.S. Pat. No. 6,896,381 issued to Pablo Benitez, Juan C. Minano, Fernando Munoz of Light Prescriptions Innovators LLC. on 2005 May 24 deals with concentrators of type RXIR, which is different than the present invention of IN type concentrators because there are no self-resonant facet surfaces.
Juan C. Minano in J. Opt. Soc. Am. A/Vol. 2 No. 11 pp 1826 shows an R3 type concentrator called a compound triangular concentrator, and in J. Opt. Soc. Am. A/Vol. 3 pp 1345 shows an RG type type Graded Refractive Index (GRIN) concentrator. Many other examples can be found and examined in detail by Prof. Roland Winston et. al. in the text “Nonimaging Optics”, ISBN: 0-12-759751-4, published by Elsever in 2005. Each of the references listed previously has one or more of the following disadvantages, while the present invention reduces or eliminates these disadvantages.
The first disadvantage of the prior art is that they use a large area of mirrored surfaces for the primary reflecting surface. These mirrored surfaces provide a failure mode for micro-cracks and mirror detachments to attack the optical system—especially over extended time periods (decades) of thermal cycling. The present invention overcomes this limitation by predominantly using Total Internal Reflection (TIR) on one or more surfaces to minimize or, in some embodiments, completely eliminate mirrored surfaces altogether.
The second disadvantage of the prior art that it does not have a built in means to control the shape of the final light spot at the output focal region. The present invention overcomes this limitation in some embodiments by shaping each facet in a slightly different way in order to facilitate a match between the shape of the focused light spot on a receiving device, located at the output aperture focal region, and the shape of said device itself—such as a photovoltaic cell.
The third disadvantage of the prior art is that it is not transparent and therefore there is no means to provide a visual representation of colors and textures that are located behind, and noncontiguous with, the concentrator to a remote observer by indirect lighting or by active light sources behind the concentrator. The prior art is therefore less aesthetically appealing to humans and is less desirable for unobtrusive integration into buildings and other platforms where the aesthetic function is required. The present invention overcomes this limitation by not using much of the concentrator's area for mirrored surfaces. In some embodiments no mirrored surfaces are needed at all.
The fourth disadvantage of the prior art again results from the fact that the concentrator is not completely transparent and therefore there is no direct way to provide diffuse light to an observer or system that is behind the lens or array of concentrator lenses. For example, such a capability is needed when the lens is integrated into a window like structure that provides both concentrated direct solar energy to a photovoltaic cell as well as dispersed natural lighting. Again, the present invention overcomes this limitation by not using much of the concentrator's area for mirrored surfaces. In some embodiments no mirrored surfaces are needed at all.
The fifth disadvantage is that the prior art is not integrated with a means for dissipating the energy from thermal heating at the concentrated light spot. The present invention overcomes this limitation by using nano-structured and transparent thermal control structures typically located on the concentrator's surface near to any optically absorptive regions that get hot from concentrated light.
The sixth disadvantage is that the prior art is typically not very thin and low profile. The present invention overcomes this limitation by providing a means to fold the optics while minimizing or eliminating the need for many, and sometimes all mirrored surfaces. Folding the optics means that the required path length for focusing the light is obtained by multiple reflections within the concentrating device instead of directly by propagation without reflections.
The seventh disadvantage is that some of the prior art exhibits a lack of broad-band capability due to material and structural dispersion of light. This is especially true for systems that employ a refractive index that changes from point-to-point in the volume of the concentrator due to intrinsic material dispersion. Certain embodiments of the present invention overcomes this limitation by minimizing refraction thereby minimizing the effects of material dispersion.
The eighth disadvantage of the prior art is that it is typically difficult to manufacture. The present invention overcomes this limitation by using low cost materials that are homogenous and which may be structurally modified and fine tuned by established machining techniques as well as a number of other process that only change the shape of a one-material optical element. Furthermore, the facets of the present invention are typically large relative to the sizes of the device. This allows for easier manufacturing and a more robust and durable lens.
The ninth disadvantage of the prior art is that it does not allow for the easy formation of arrays of lenses. The present invention overcomes this limitation by providing a structure that has a facet morphology that may act as mechanical spars, which provides an integrated stiffener to keep an array of lenses optically flat even under external loads—such as from high winds or a human walking on meter-scale tiles of the concentrators.
The tenth disadvantage of the prior art is that most optical elements are rotationally or transitionally symmetric, which leads to rotational and translational skew invariance. This invariance tends to limit the concentration performance beyond that suggested by the conservation of etendue. Certain embodiments of the present invention break the rotational or translational symmetry by having different facet shapes at non periodic facet locations for the purpose of shaping the focal spot to a desired shape or for simply improving concentration performance.
The eleventh disadvantage of the prior art is that when curved facets are employed they tend to be small structures relative to the size of the largest dimension of the device and to incorporate very acute angles at their apex thereby making them difficult to manufacture and fragile. In the present invention the facets are a large fraction of the largest dimension of the device. This helps improve the manufacturability of the device.
The twelfth disadvantage is that the prior art can not provide both concentrated light and dispersed light to both a high efficiency photovoltaic cell that uses only directly concentrated light and to thin-film photovoltaic cells for dispersed light simultaneously. The present invention can because it is transparent to diffuse light.
While all of these disadvantages may not exist for any one particular prior art design all of the previously mentioned sources of prior art do suffer from one or more of the above stated deficits. However, each embodiment of the present invention incorporates the majority of the advantages alluded to above and discussed in detail throughout this document. These advantages all stem from the unique self-resonant curved facets, which provide a means for light redirection by total internal reflection.
In summary, there are a large number of prior art devices that are currently disclosed and based on Refraction (R), Reflection (X), Gradient (G) index, and total internal (I) reflection for concentrating or collimating light energy. However, none of these teaches or anticipates the present invention of a concentrating optical device, which predominantly uses multiple (N) efficient total internal reflections IN in a high dimensionality phase-space design. The current invention incorporates large non-linear facets that provide very large optical efficiencies, large angles of acceptance, shaping of the focal spot, manufacturability and a compact design. Thus the prior art is seen to have multiple deficiencies, which are addressed and overcome in the present invention.